The chief aim of these experiments is to determine the underlying basis for the length-tension relation in cardiac muscle. The methods entail the control of sarcomere length or force in isolated heart muscle. A light diffraction technique will be used to measure directly sarcomere length during contraction in thin preparations from the right ventricles of rats. Programmed stretch or release of the muscle preparation will allow suitable control of sarcomere length or tension. In addition, sarcomere length will be controlled directly by servo controlled feedback techniques. First, the force-velocity property of the cardiac sarcomeres will be measured by shortening the sarcomere at a predetermined rate. Developed force will be related to the actual velocity of controlled shortening at a specified myofilament overlap. The shape of the force velocity relation will be compared to those obtained under a variety of shortening conditions to determine the precise influence of motion upon the contractile behavior of the cardiac sarcomere. Next, the length I want to test whether the shape of the length-tension diagram is a fundamental property of myofilament overlap alone. The isometric length tension diagrams will be constructed from the dynamic measurements obtained from the force velocity data. Importantly, such an approach does not eliminate uncertainties due to variations of shortening or initial sarcomere length. Second, I will obtain the sarcomere length tension relations under conditions where contractile activation is steady. Here, data obtained from a control "twitch" contraction will be compared to that obtained from tetanically contracting heart muscle, in order to see if there is a fundamental change in shape of the length tension relation.